<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript" src="http://code.jquery.com/jquery-git.min.js"></script>
    <style type="text/css">
        table, thead, th, tr, td{
            border: 1px solid;
        }
    </style>
</head>
<body>
<table id="table" style="border:1px solid;">
    <thead>
    <th>th1</th>
    <th>th2</th>
    <th>th3</th>
    <th>th4</th>
    </thead>
    <tbody>
    <tr>
        <td>td</td>
        <td>td</td>
        <td>td</td>
        <td>td</td>
    </tr>
    </tbody>
</table>
<script type="text/javascript">
    var ths = $('#table thead th');
    for(var i=2;i<ths.length;i++){
        ths[i].remove();
    }
    ths[1].colSpan=3;
    ths[1].innerHTML='';

</script>
</body>
</html>